Such a process is known from GB-A-1286,427. In that patent publication a process is described for removing cyclohexanone oxime from caprolactam by supplying gaseous sulphur dioxide to and dissolving it in the cyclohexanone oxime-containing caprolactam mixture at a temperature of 70.degree.-170.degree. C., the sulphur dioxide being dissolved to a concentration of at least 1 mole sulphur dioxide per mole cyclohexanone oxime. After completion of the reaction of the sulphur dioxide with the remaining cyclohexanone oxime, the excess of sulphur dioxide is removed by evaporating it from the reaction mixture under reduced pressure or by supplying inert gases to the reaction mixture. The purified caprolactam is subsequently recovered by distillation.
A disadvantage of such a process is that the product formed by the reaction of sulphur dioxide with a ketoxime or aldoxime is a process-alien substance, which must therefore be removed from the process, so that a potential amount of amide to be formed, viz. the non-rearranged ketoxime or aldoxime, is withdrawn from the process.
It is generally known in the art how to produce amides, for instance lactams such as s-caprolactam, by means of a homogeneously catalytic Beckmann rearrangement of ketoximes or aldoximes such as, for instance, cyclohexanone oxime. This rearrangement is effected by treating the ketoxime or aldoxime with strong acids such as, for instance, sulphuric acid, oleum, chlorosulphonic acid, hydrogen fluoride, polyphosphoric acid, phosphorus pentachloride and the like. When using, for instance, sulphuric acid or oleum, a sulphuric acid-amide complex is obtained after the rearrangement, upon which the desired amide must be recovered by neutralizing the reaction mixture with usually ammonia water, in which process a large amount of ammonium sulphate is obtained as byproduct.
Another and more favorable process is the conversion of a ketoxime or aldoxime into the corresponding amide by means of a heterogeneously catalytic Beckmann rearrangement using a solid acid or neutral catalyst, for instance a rearrangement in the gas phase or in the liquid phase. Examples of a solid acid or neutral catalyst to be used are boric acid on a carrier, such as silica or alumina, and crystalline silicas, such as silicalite I (silicon-rich MFI (Mobil Five, also known as ZSM5)) and silicalite II (silicon-rich MEL (Mobil Eleven, also known as ZSM11)), as well as an acid ion exchanger or (mixed) metal oxides and the like. An advantage is that in such a process no ammonium sulphate is formed as byproduct.
In such Beckmann rearrangement processes, however, an incomplete conversion of the ketoxime or aldoxime may occur, causing a certain amount of non-reacted ketoxime or aldoxime to leave the reactor along with the amide formed. On the other hand, a complete removal of the ketoxime or aldoxime from the oxime-containing amide mixture is highly desirable in order to obtain a high degree of amide purity in the preparation of the amide and in order to avoid disturbances in the further amide preparation process. Now, the separation of this oxime from the oxime-containing amide mixture by means of physical separation techniques is very difficult and can be realized only at high costs (reference may be made to GB-A-1,286,427).